There are the following requirements to the stated polymer spherical particles: they must be mechanically strong, homogeneous by their chemical composition, have the certain size and size distribution.
There is a number of methods of production of polymer microspheres with the size of 1-10 micron, containing 0.5-80% of linking agent.
These microspheres can be produced by suspension polymerization.
There is a method of production of macroporous nonionic sorbents by suspension polymerization of pure or technological divinylbenzene in presence of a radical initiator and a porogene at heating. At that, azeotropic mixtures of aromatic and aliphatic compounds with the common boiling temperature of at least 80° C. are used as porogenes. (application RU 93002986, 1996).
However, the stated method of dispersion of the monomer and the initiator, soluble in the monomer, in the uninterrupted phase in presence of steric stabilizer allows to produce highly cross-linked particles with wide size distribution.
The co-polymer microspheres with the size of 1-10 micron with narrow size distribution are produced at one stage by the method of dispersion polymerization.
According to this method, the reaction starts in initially homogeneous solution, and then the polymer particles are distilled into separate phase and grow further to their final size. The process is performed in presence of steric stabilizers. However, this method does not allow to produce microspheres with the content of linking agent more than 10-15% (J. Appl. Polymer Sci. 2008. Vol. 107, N 6. P. 3909-3916.).
Microspheres with the content of linking agent more than 10-15% can be produced with two-stage method of radical polymerization, suggested by Ugelstad, according to which the radical microspheres, produced beforehand and stabilized in the uninterrupted phase, are treated with a substance, simplifying their inturgescence, and then with the linking agent. As the result of polymerization, the microspheres containing linear polymer are formed. However, these microspheres are chemically heterogeneous and in presence of appropriate solvents the linear polymer can be easily taken out of them, forming the macropores (J. Polymer Sci.: Part A Polymer Chemistry. 1994. Vol. 32, P. 2577-2588). Consequently, these microspheres are not as strong as their highly cross-linked analogues, and cannot be applied in situations where high mechanic strength is required, for instance, at production of columns for ultra-productive liquid chromatography (UPLC).
Highly cross-linked monodisperse microspheres with the size of about 1-8 micron and containing 45-80% of linking agent, can be produced by the method of precipitative polymerization. The method comprises the performance of polymerization in the environment, dissolving the initiator, monomer and linking agent. At the early stage of the reaction the oligomers form a separate phase and then microspheres, which gain colloidal stability due to mechanic strength and salvation of free linear oligomers by the solvent, as their degree of cross-linking increases. According to this method, the reaction can be performed mainly in solvents with the properties as good as those of teta-solvents in relation to polymer, and monomers containing at least 45% of linking agent should be used. The method allows to produce chemically homogeneous polymer microspheres, suitable to use as matrix for sorbent production in such fields as UPLC.
The method of precipitative polymerization is described in a number of works taking as the example mainly the polymerization of technology divinylbenzene 55 (DVB-55) or DVB-80, containing respectively 55 or 80% of linking agent and its mixtures with functionalized monomers, such as 4-chlorine methyl styrene, glicidyl acrylate and others. In the work (Journal of Polymer Science: Part A: Polymer Chemistry. 2004. Vol. 42. P. 3967-3974) the method of polymerization of divinylbenzene in acetonitrile at the temperature of 70° C. in presence of azoisobutyronitrile. The monomer is taken in the amount of 2-15% of the solvent volume; the polymerization is performed in a water bath, equipped by a shaker, which allows to produce monodisperse microspheres with the diameter of 2.9-3.8 micron, depending on the ratio of reagents.
There is also the method of precipitative polymerization of divinylbenzene in acetonitrile at the temperature of 30° C., where the radical decomposition of isobutyronitrile is initiated photochemically (U.S. 20090326081 A1, 2009). Shaking of the mixture is performed by a special rotatory device, which rotates the vessels with reaction mass around its horizontal centerline. The method allows to use solutions of 2-10% monomer of the solvent volume and produce the microspheres with average diameter of 1.4-3.9 micron, depending on the concentration of the monomer and the initiator.
The common disadvantage of the stated methods of polymerization is the impossibility to regulate the size of produced microspheres, as their size is closely connected with other process parameters, which finally determine the outcome of the polymer. According to the examples given in the existing methods, the size of polymer microspheres decreases as the monomer concentration is reduced and, respectively, grows when it is increased. Therefore, the size of microspheres depends on the outcome of the polymer from a volume unit of the reaction mass, which reduces economic efficiency of the existing methods at production of the small size microspheres. Similarly, the smaller size microspheres can be reached by reducing the concentration of the initiator, which also determines the dependence between the polymer outcome and microsphere size.
There is also a method of production of polymer microspheres based on divinylbenzene 50 or 80 and its co-polymers, where functionalized styrenes, such as chlorine methyl styrene and acrylates, mainly glicidyl metacrylate, are used as co-monomers. The process is performed in acetonitrile at basic monomer load of 2% of the reaction mixture volume. 2,2-Azobisisobutyronitrile in the concentration of 0-20% is used as the initiator, the reaction mixture is shaked by smooth rotation of vessels around the horizontal centerline of the mixing device. (Patent U.S. Pat. No. 5,599,889, 1997)
The method of the present invention regulates the size of microspheres using the binary mixtures of acetonitrile and other solvents as reaction environment for polymerization. However, this makes the reactivation and purification of the solvents at repeated use more difficult and, more important, leads to the deterioration of size distribution of the microspheres, as shown in the examples of the use of acetonitrile and propionitrile mixture and acetonitrile and water mixture.
The closest analogue of the suggested invention is the method of production of monodisperse co-polymer microspheres of divinylbenzene, described in (Li K. Functional Cross-linked Polymer Microspheres, 1994; http://digitalcommons.mcmaster.ca/opendissertations/2413).
It is also based on the precipitative polymerization of divinylbenzene and related compounds in homogeneous solution. The typical process comprises the use of 2-4 vol. % DVB-55 or DVB-80 solution in pure acetonitrile and 2,2′-azobisisobutyronitrile (AIBN) as the initiator in the amount of 2% wt. (in relation to the total amount of the monomer).
The polymerization is performed in the polymer vessels with the volume of 30 to 1000 ml, rotating them around the long centerline to prevent subsidence if the formed microspheres and further coagulation. The reaction temperature is increased to 70° C. in 2 hours, and then kept 70° C. during 24 hours. The conversion is 40 to 60%, which is determined by low concentrations of the monomer and the initiator.
According to the known method, the substituted styrenes, mainly chlorine methyl styrene in the concentration of 0-20% in relation to the total amount of the monomer, acrylic monomers, mainly glicidyl metacrylate mixed with divinylbenzene, containing at least 50% of linking agent, can be used as co-monomers.
Azo-initiators, mainly 2,2′-azobisisobutyronitrile in the concentration of 0-20%, initiators of peroxide type, mainly benzoyl peroxide, can be used as initiators.
It is suggested to use acetonitrile and its mixtures with water (in the concentration of 1-7% wt. of the reaction mixture volume), with propionitrile (in the concentration of 1-60% wt. of the reaction mixture volume), with toluol as porogeneous agent (in the concentration of 40% wt. max of the reaction mixture volume) as solvents.
To reduce the size of formed cross-linked microspheres it is suggested to use polymer stabilizers. It is demonstrated, that the use of polyvinyl pyrrolydone in the concentration of 16 g/l to 48 g/l allows to reduce the size of microspheres from 2.68 micron to submicron size at initial monomer load of 10 wt. % of acetonitrile volume.
The disadvantage of known method is the fact that polymer stabilizers are effectively sorbed on the surface of the microspheres, and, as the result, their desorbtion rate is rather low. The existence of multiple centers, capable of transferring the chain, leads to the intrusion of the stabilizer into the microspheres' structure, which results in their chemical heterogeny. The influence of inoculation of the stabilizer on the microspheres' structure is shown in the polymerization of divinylbenzene in presence of poly(styrene-bulk-4-vinylpyridine) (Lee J. M., Saikia P. J., Lee K.//Macromolecules. 2008.Vol 41, N 6. P. 2037-2044).